Vinyl chloride resin granules which have conventionally been employed are produced by suspension polymerization or emulsion polymerization of vinyl chloride or a vinyl chloride-based monomer mixture in the presence of a surface active agent to obtain an aqueous resin dispersion, followed by granulation by spray drying of the aqueous resin dispersion.
Where the vinyl chloride resin granules are used for, e.g., preparing paste, the granules are mixed with a plasticizer to form a sol (called sol formation), and the resulting sol (paste) is molded and hardened by heating (called gelation) to obtain a final cured product. If the vinyl chloride resin granules have insufficient sol-forming properties or if the sol contains large particles which remain undispersed, the workability in paste molding is deteriorated or the properties of the final product are degraded. Thus, in sol formation, the vinyl chloride resin used have been finely ground. Therefore, the particle shape of the vinyl chloride resin has been considered to have no direct influence on the final product after they have been ground for preparing paste and has received very little attention.
However, as a result of studies on the shape of vinyl chloride resin particles, the inventors have found that particles of various irregular shapes, e.g., hollow particles with large holes and broken hollow particles, do not necessarily have satisfactory particle characteristics, such as flow characteristics. Moreover, these particles have a low bulk specific gravity, a small weight per unit volume, and thus have an increased cost of transportation.
Further, a vinyl chloride resin having been finely ground for preparing paste causes dusting when bags of the resin are opened and affect the working environment and cannot be automatically weighed and fed.
In order to solve the above-described problems, attempts have been made to obtain a granular vinyl chloride resin which can be used for preparing paste without requiring fine grinding. For example, it has been proposed to lower the temperature of drying air at the time of feeding and spraying in the spray drying of the aqueous resin dispersion to obtain granules having excellent particle characteristics and satisfactory sol-forming properties as disclosed in JP-B-57-5815 (the term "JP-B" as used herein means an "examined Japanese patent publication") and JP-A-60-120726 (the term"JP-A" as used herein means an "unexamined published Japanese patent application").
However, reduction of the drying air temperature reduces the drying rate, resulting in an increase in the water content remaining in the granules. A gel obtained from a sol prepared from granules having a high residual water content provide molded articles having poor surface properties.
When the average particle size of the granules is controlled to be as small as about 20 .mu.m as suggested in JP-B-57-5815, the drying rate is improved, and the residual water content can be decreased but, in turn, the particle characteristics are deteriorated. On the other hand, when the average particle size is controlled to be as large as about 80 to 100 .mu.m as suggested in JP-A-60-120726, the particle characteristics are improved, but the residual water content becomes higher, and requires an extra drying step.
Further, these processes of granulation still involve a problem that the sol contains a large number of undispersed granules. In addition, sol-forming properties of the granules are further reduced during storage, and the proportion of the undispersed granules in the resulting sol is increased, ultimately becoming unsuitable for use in paste molding.
To prevent reduction of sol-forming properties, it has been proposed to add glycol ethers or nonionic surface active agents as disclosed in F. Ponizil, Kunststoffe, Vol. 64, p. 3 (1974). These additives, however, sometimes deteriorate quality of the final products and are, therefore, unsuitable for general use.